Monitoring Respiratory Status

1
Monitoring Respiratory Status
2
Pulse Oximetry

• Measures oxygen saturation
• Documents peripheral oxygen availability

3
Oximetry History

• Became standard of care in the 1980s
• 1935 Carl Matthes
• first oximeter
• 1930s J.R. Squires
• self calibrating oximeter

4
Oximetry History (Contd)

• 1940s Glen Milliken
• aviation oximeter
• 1951
• First reported use of oximetry in the operating
  room

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Has pulse oximetry improved the outcome of
patients receiving anesthesia?
6
Improved Safety

• Research is not ethical
• Closed claim studies
• capnography
• pulse oximetry
• Insurance rates have dropped

7
What information can be obtained from the pulse
oximeter?
8
Physics Engineering

• Lambert-Beer law
• concentration of a liquid is related to the
  amount of light that will pass through it
• Oxygenated hemoglobin absorbs a different
  wavelength of light than does deoxygenated blood

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Cyanosis

• Is cyanosis a reliable clinical sign?
• What is required for cyanosis to occur?
• At what saturation will the average patient
  become cyanotic?

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Light Absorption

• Tissues
• Nonpulsatile vessels
• Pulsatile arterial blood

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Pulse Oximeter Wavelengths

• Red (660 nm)
• absorbed by unoxygenated hemoglobin
• Near infrared (940 nm)
• absorbed by oxygenated hemoglobin

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Cause of False Readings

• Accurate only with saturation 80
• most accurate 95
• shows trends at low saturation
• Abnormal hemoglobin

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False Readings (Contd)

• Intravenous dyes
• Diminished pulse
• Movement of finger
• Ambient light
• Nail polish

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Abnormal Hemoglobin

• Carboxyhemoglobin
• false high reading in carbon monoxide patients
• Methemaglobin
• reads 85 regardless of actual saturation
• Fetal hemoglobin
• little effect on pulse oximetry

15
Dyes

• Methylene blue
• Indocyanine green
• Indigo carmine
• Acrylic finger nails
• nail polish

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Capnometry vsCapnography
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Monitoring Expired CO2

• Capnometry gives numerical value
• Capnography gives wave form

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Monitoring ETCO2

• Confirms the movement of air in and out of the
  lungs
• Assumed to reflect alveolar CO2
• Assumed to indicate adequacy of ventilation

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Monitoring ETCO2 (cont)

• Better indicator of ventilation
• Measures high point of the expiratory plateau
• Normally less than the PaCO2
• Normal gradient about 5-8

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Sampling Sites

• Main stream
• measuring device is in line
• measures only CO2
• patient must be intubated

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Sampling Systems

• In line
• Side stream

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Sampling Sites (Contd)

• Side stream
• takes sample to processing machine
• removes gas from circuit

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CO2 Increases with

• Hypoventilation
• Malignant hyperthermia
• Sepsis
• Rebreathing
• Bicarbonate administration
• Insufflation of CO2

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CO2 Decreases with

• Hyperventilation
• Hypothermia
• Low cardiac output
• pulmonary embolism
• Circuit disconnect
• Cardiac arrest

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Describe Wave Forms representing the following

• Normal wave form
• COPD
• Inadequate neuromuscular relaxation
• Unequal lung emptying

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Describe Wave Forms (Contd)

• Restrictive lung disease
• Esophageal intubation
• Malignant hyperthermia
• Cardiac arrest
• Pulmonary embolism

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Clinical Uses of Capnography

• Detection of untoward events
• Maintenance of normocarbia
• Weaning from mechanical ventilation
• Evaluating effectiveness of CPR

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Monitoring Anesthetic Gases

• What types of gases are present?
• What are their concentrations?
• partial pressure
• volume percent

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Anesthetic Gas Monitoring

• What types of gases are present?
• What are their concentrations?
• partial pressure
• volume percent

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Sampling Systems

• In line
• Side stream

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Mass Spectrometry

• Gas enters high vacuum area
• Bombarded by electron beam
• Charged particles passed over strong magnet

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Mass Spectrometry (Contd)

• Different components are deflected according to
  their chemical composition
• Specific collectors measure composition

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Mass Spectrometry Display

• Assumption that the sum of the gases 100
• Calibrated to ambient pressure minus 47 mm/Hg
• greater chance for error with mm/Hg
• percentages remain accurate

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Infrared Analyzers

• Measures energy absorbed from narrow band of
  wavelengths of infrared light passing through a
  gas sample

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Infrared Analyzers (Contd)

• Molecules that absorb energy
• carbon dioxide
• nitrous oxide
• water vapor
• volatile anesthetics

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Infrared Analyzers (Contd)

• Molecules that do not absorb energy
• oxygen
• argon
• nitrogen
• helium
• xenon

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Ramon Spectrometer

• When light strikes gas molecules, most of the
  scattered energy is absorbed and re-emitted in
  the same direction

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Ramon Spectrometer (Contd)

• Small amount of light is scattered and detected
  by optical detection system
• Both polar non-polar molecules are detected

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Ramon Spectrometer (Contd)

• Monoatomic gases lacking intermolecular bonds are
  not detected
• helium/xenon/argon

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Neurological Monitoring
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Common Parameters Monitored

• Cerebral blood flow
• EEG
• SSEP
• EMG
• Wake-up test

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What to look for?

• Blood flow
• Too much
• Too little
• Location of nerve
• Integrity of nerve

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Monitoring Cerebral Blood Flow

• Level of consciousness
• Blood pressure
• Intracranial pressure
• Jugular venous oxygen content
• Radioactive venous washout
• Transcranial doppler

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Essentials of Neurological Monitoring

• Must assess function of area at risk
• Operator must understand pathways assessed
• Monitoring should be continuous
• Minimal interference
• Strict quality control

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Preoperative Assessment

• Essential to understand baseline status
• Patient teaching

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Wake-up Test

• Test neurologic function following reversible
  surgical manipulation
• Movement must not cause damage
• Patient is allowed to awaken
• Amnesia must be maintained

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Wake-up Test

• After awakening, patient follows verbal commands
• Evaluates corticospinal tracts (thoracic)
• Response to painful stimuli
• Lumbar cord function

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Disadvantage of Wake-up

• Test is intermittent
• not assessed as distraction is applied
• gray matter damaged quickly
• white matter damaged slowly
• Surgery is interrupted

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Wake-up disadvantage

• Risk of patient injury
• Danger of recall
• In spite of disadvantages, wake-up test is still
  used successfully

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Awake Clinical Observation

• Carotid endarterectomy
• Resection of seizure focus
• Resection of brain tumor
• Non-neurological surgery on patient with head or
  neck trauma
• Patient selection is critical

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Sedation / Analgesia

• May mask neurological changes
• Propofol infusion
• Rapid on and off
• Impairs continuous monitoring
• Risk loss of airway

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CBF Monitoring

• Brain has high metabolic rate
• Oxygen supply is critical
• Even brief interruption of CBF can cause
  perminent brain damage

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Cerebral Blood Flow

• Autoregulation attempts to maintain constant CBF
• CBF constant with CPP in the range of 50-150
  mm/hg
• At extremes, CBF related to MAP

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Cerebral Perfusion Pressure

• CPP MAP - ICP or CVP (whichever is higher
• Unless ICP is measured, CPP can only be estimated
• If BP is constant, CPP falls as ICP increases

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Measuring ICP

• Ventricular catheter
• Subdural bolt
• Lumbar CSF catheter
• Scanning techniques

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Ventricular Catheter

• Small burr hole
• Catheter into lateral ventricle
• Connected to transducer
• note do not flush
• This is the most accurate technique
• Fluid can be removed to release pressure

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Subdural Bolt

• Small drill hole
• Dura open
• Hollow fluid filled bolt rests directly against
  the brain
• Less Accurate than ventricular cath.
• Can not remove fluid

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Lumbar CSF Catheter

• Place epidural type catheter
• Attach to transducer
• Correlates with ICP if zeroed at level of the
  foramen of monroe
• CSF blockage will alter accuracy

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Scanning Techniques

• CT or MRI may show edema or distended ventricle
• Does not provide quantative number
• Does not substitute for ICP monitor

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Measuring Cerebral Blood Flow

• Direct
• Radioactive scan
• Indirect
• Transcranial doppler

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CBF Scan

• Inject radioactive xenon into carotid artery
• Measure radioactivity and washout
• Not continuous
• Not used in the operating room

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Transcranial Doppler

• Direct, continuous, non-invasive
• Ultrasound waves to basal artery
• Assumed that blood velocity is related to CBF

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Uses of Transcranial Doppler

• Carotic endarterectomy
• Cardiopulmonary bypass
• Detection of vasospasm
• Confirmation of brain death

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EEG

• 10-20 electrodes placed on scalp
• Each channel is electrical activity between 1
  pair of electrodes
• Requires continuous monitoring by trained
  technician
• Even 1-2 channels may be helpful

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Types of EEG waves

• Alpha
• Beta
• Delta
• Theta

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Alpha Waves

• Occipital area of the brain
• Patient is alert, relaxed with eyes open
• May be seen in light plane of anesthesia

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Beta Waves

• Mental concentration in the awake patient
• Seen with low doses of sedatives or hypnotic
  drugs
• Abolished with deep anesthesia or ischemia

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Delta Waves

• Deep sleep or deep anesthesia
• Ischemia
• Drug overdose
• Severe mental derangements

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Theta Waves

• Commonly seen during general anesthesia
• May be seen in same pathological states as delta
  waves

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Using the EEG

• Equipment must be properly applied
• Set acceptable parameters
• Learn anticipated changes
• If possible, have EEG technician monitor the
  patient

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Uses of the EEG

• Determine cerebral ischemia
• need for shunt placement
• hypotension
• barbiturate suppression of CMRO2

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Drug effects on the EEG

• Low dose of GA with nitrous oxide
• active EEG with alpha and beta
• Deep anesthesia with volatile agent
• Similar pattern to ischemia
• Very deep anesthesia
• flat EEG

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Drug Effects

• Drug bolus may resemble ischemia
• Steady state anesthesia pruduces stable EEG

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What are somatosensory Evoked Potentials?
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The Somesthetic Nervous System

• Extends throughout peripheral and central nervous
  system
• Peripheral nerves
• Spinal cord
• Subcortical structures
• Cortical structures

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Somesthetic Sensory System

• Carries sensory information
• Vibration
• Proprioception
• Light touch

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SSEPs

• Peripheral nerve stimulated
• Response recorded proximal to stimulation and at
  cerebral cortex
• Should monitor both sides of the body even though
  surgery is only on one side
• Each stimulus should reach brain

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SSEP Helpful For

• Spinal surgery
• Thoracic aneurysm
• Cerebral blood flow
• cerebral aneurysm
• carotid endarterectomy

79
Anesthesia may alter SSEP

• Hypnotics and volatile agents increase latency
  and decrease amplitude
• Muscle relaxants have no effect
• Narcotics have minimal effects if given in
  moderate doses

80
Other Factors Affecting SSEP

• Hypothermia
• Cold irrigation
• Hypoxemia
• Sudden changes in PaCO2 or anesthetic agent

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Minimizing the effects of anesthesia on SSEPs

• Do not change anesthetic concentration during
  critical times
• Monitor area of brain not at risk
• Use cervical response rather than cortical
  response when possible
• Use favorable anesthetic technique

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Motor Evoked Potentials

• Similar to SSEP
• Motor cortex in brain is stimulated
• Descending motor pathways conduct the impulse
• Response recorded by EMG

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Clinical Application of MEP

• Not commonly used
• Problems
• pathways yet to be determined
• experience limited
• May not be better than SSEP
• profound effects by anesthetic agents

84
Brainstem Auditory Evoked Potentials
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BAEP

• Monitors the 8th cranial nerve
• Auditory stimulus in the ear
• Scalp electrodes record response

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Clinical use of BAEP

• Monitors 8th cranial nerve function during
  surgery of the posterior fossa
• Used during acoustic neuroma surgery

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Advantage of BAEP

• Specific function of 8th nerve
• Less affected by hypothermia or anesthetic agents
  than other EPs

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Visual Evoked Potentials
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Visual Evoked Potentials

• Not commonly used
• Assesses function of the optic nerve
• Goggles emit bright light through closed eyes.
• Stimulus detected by the brain

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Use of VEP

• Monitors optic nerve function in operations near
  the optic nerve or the chiasm
• pituitary surgery
• meningiomas that compress the optic nerve

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Facial Nerve Monitoring
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Facial Nerve Observation

• Simple method of assessment
• Surgeon observes for muscle twitch when nerve is
  touched
• Twitch may not be readily visible
• Muscle relaxants abolish the twitch

93
Facial Nerve Monitoring

• Needle placement
• Orbicularis Oculi muscle
• Orbicularis Oris muscle
• EMG activity is recorded
• Muscles twitch if facial nerve is touched

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Advantage of Facial Nerve Monitoring

• Audible signal when nerve is touched
• More reliable than visual observation
• Note The patient must have a twitch for this
  technique to work